Missing link spotted in planet formation

Astronomers detect planets as well as dusty leftovers

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At left, a visible-light image shows a debris disk around the red dwarf star AU Microscopii. At right, a false-color view from Hubble traces a planetary debris disk encircling the star HD 107146, a yellow dwarf star very similar to our sun. HD 107146 is much younger, however — dated at 30 million to 250 million years old, compared with the 5-billion-year age of the sun.

For several years, scientists have been detecting planets around mature stars and, separately, imaging dust disks around younger stars. A strong theory has developed that planets form from these disks of material, leftovers of the star formation process.

In other investigations, lumpy objects in young star systems have been seen, and researchers figure they're planets in the making.

However there has been a glaring gap in the observations: Mature stars harboring both planets and disks have remained elusive, perhaps simply because the disks have thinned out to almost imperceptible levels.

New observations have begun to fill in the missing link.

NASA's Spitzer Space Telescope has detected dust disks around mature, sunlike stars known to have planets. The dust is in the outlying reaches of each system and is presumed to result from collisions between objects, as occurs in our own solar system's Kuiper Belt, out beyond Neptune.

Theory seems to be on track
The findings suggest that the current theory tracing planet formation, including here in our solar system, are on track. The theory goes like this: A star is born amid a cloud of gas and dust. Leftovers swirl into a relatively flat disk that orbits the rotating star. Dust and chemicals collect and build rocky and icy objects, which collide. Some stick to make comets, asteroids and planets. Other collisions later on continue to produce dust — the kind of dust that Spitzer detected.

The new results "fit into theoretical picture that we've built up about how stars and planetary system form," said Charles Beichman of NASA's Jet Propulsion Laboratory, lead author of the study.

"Spitzer has established the first direct link between planets and disks," Beichman said. "Now, we can study the relationship between the two."

Hubble spots dust disks
Separate observations by the Hubble Space Telescope reveal significant dust disks around young stars, one of them between 50 million and 250 million years old. Importantly, that star is similar to our sun.

"The new Hubble image gives us the best look so far at reflected light from a disc around a star the mass of the sun," said Hubble study leader David Ardila of Johns Hopkins University. "Basically, it shows one of the possible pasts of our own solar system."

Both results were presented Thursday afternoon in a NASA teleconference with reporters.

Alycia Weinberger, an astronomer at the Carnegie Institution of Washington, suggests that the dust around a star is like bricks at a construction site. Previous observations had revealed the bricks, she said, and other observations had shown the completed houses, but the two hadn't been found at the same site.

"The new observations close the loop. They show us both the planets and the disks," said Weinberger, who was not involved in the work.